The present invention relates generally to a process for manufacturing sulfuric acid, and more specifically, to a process for manufacturing relatively dilute sulfuric acid from sulfur-bearing materials using high temperature pressure leaching processes and recovering metal values from the sulfur-bearing materials.
Hydrometallurgical treatment of copper containing materials, such as copper ores, concentrates, and the like, has been well established for many years. Currently, there exist many creative approaches to the hydrometallurgical treatment of these materials. The recovery of copper from copper sulfide concentrates using pressure leaching promises to be particularly advantageous.
The mechanism by which pressure leaching releases copper from a sulfide mineral matrix, such as chalcopyrite, is generally dependent on temperature, oxygen availability, and process chemistry. In high temperature pressure leaching, typically thought of as being pressure leaching at temperatures above about 200xc2x0 C., the dominant leaching reaction in dilute slurries may be written as follows:
4CuFeS2+4H2O+17O2xe2x86x924CuSO4+2Fe2O3+4H2SO4xe2x80x83xe2x80x83(1)
During pressure leaching of copper sulfide concentrates, such as chalcopyrite containing concentrates at medium temperatures (e.g., at temperatures in the range of between about 140xc2x0 C. to about 180xc2x0 C.), however, a significant fraction of the sulfide converts to elemental sulfur (Sxc2x0) rather than sulfate (SO4xe2x88x922). According to the reaction:
4CuFeS2+4H2SO4+5O2xe2x86x924CuSO4+2Fe2O3+8Sxc2x0+4H2Oxe2x80x83xe2x80x83(2)
For example, experimental results show that at about 160xc2x0 C. and about 100 psi oxygen overpressure in the pressure leaching vessel, from about 60 to about 70 percent of the sulfur in the super-finely ground copper sulfide concentrate is converted to elemental sulfur, with the remainder being converted to sulfate.
Elemental sulfur is a hydrophobic substance. In the pressure leaching process slurry, under certain temperature and solution conditions, sulfur has a tendency to agglomerate. Moreover, molten elemental sulfur becomes highly viscous us at elevated temperatures. For example, the viscosity of molten sulfur increases from less than 100 centipoise at 150xc2x0 C. to more than 90,000 centipoise at 185xc2x0 C. As such, the molten sulfur may tend to encapsulate metal values in the process slurry, including precious metals and unreacted metal sulfides, and/or stick to various parts of any apparatus in which processing operations on the molten sulfur are performed. Encapsulation of the metal values, for example, copper, precious metals and the like, tends to make subsequent recovery of such metal values extremely difficult using conventional processing techniques. As discussed in applicant""s co-pending application entitled xe2x80x9cMethod for Recovery of Metals From Metal Containing Materials Using Medium Temperature Pressure Leachingxe2x80x9d filed Jul. 25, 2001 and assigned U.S. Ser. No. 09/915,105, the subject matter of which is hereby incorporated herein by reference, while pressure leaching under medium temperature conditions offers many advantages, prior medium temperature pressure leaching processes characteristically have suffered from incomplete metal (e.g., copper) extraction resulting from either passivation of the metal sulfide particle surfaces or by the metal sulfide particles becoming coated with molten elemental sulfur. As discussed in greater detail in applicant""s co-pending application, proper control of such pressure leaching processes, as described therein, enables the formation of elemental sulfur in addition to the desired metal recovery (e.g. copper). However, recovery of metal values that may be contained in the elemental sulfur-containing residue, such as, for example, precious metals, may be difficult with use of conventional techniques, and as such they may be lost. Moreover, if the acid produced by such processing techniques could not be used at the site where the recovery was performed, costs would be incurred in connection with transportation of the residue or handling of the acid. An effective and efficient method to manufacture sulfuric acid from sulfur-bearing material, particularly elemental sulfur-containing residue resulting from pressure leaching operations operated at medium temperatures (e.g., about 140xc2x0 C. to about 180xc2x0 C.) is needed. Moreover, an effective and efficient method to enhance recovery of any metal values encapsulated within the sulfur-bearing material would be advantageous.
While the way in which the present invention addresses the deficiencies and disadvantages of the prior art is described in greater detail hereinbelow, in general, according to various aspects of the present invention, a process for manufacturing sulfuric acid includes pressure leaching of sulfur-bearing materials, preferably at high temperatures, not only to facilitate the recovery of a sulfuric acid solution, but also to enhance recovery of metal values contained in the sulfur-bearing materials. The acid produced, preferably a relatively dilute sulfuric acid solution advantageously can be used in other metal extraction processes, often with significant cost savings.
As will be described in greater detail hereinbelow, the methods and processes of the present invention are particularly suited for use in connection with sulfur-bearing materials comprising residues from pressure leaching operations, such as, for example, those operated at medium temperatures (e.g., about 140xc2x0 to about 180xc2x0 C.).
In accordance with an exemplary embodiment of the present invention, a process for manufacturing sulfuric acid from sulfur-bearing materials generally includes the steps of: (i) providing a feed stream containing a sulfur-bearing material, and (ii) subjecting the sulfur-bearing material feed stream to high temperature pressure leaching in a pressure leaching vessel, optionally in the presence of a suitable dispersing agent. In accordance with a preferred aspect of this embodiment of the invention, the sulfur-bearing material feed stream comprises residue from medium temperature pressure leaching of a copper sulfide mineral, such as chalcopyrite or a blend of that residue combined with elemental sulfur. In accordance with a further preferred aspect of this embodiment of the invention, the use of a dispersing agent during pressure leaching may aid in alleviating processing problems caused by the high viscosity and hydrophobic nature of elemental sulfur at higher temperatures (e.g., above about 160xc2x0 C.
In accordance with a further aspect of this embodiment of the present invention, metal values contained in the sulfur-bearing material feed stream are liberated from the elemental sulfur residue during pressure leaching, during which the elemental sulfur is converted to sulfuric acid, and then separated from the resultant acid stream and subjected to metal recovery processing. Such metal recovery processing may include precious metal recovery.
The present inventors have advanced the art of copper hydrometallurgy by recognizing the advantages of not only producing a sulfuric acid solution from sulfur-bearing materials, such as the elemental sulfur by-product of medium temperature pressure leaching of copper sulfide minerals, but also of enabling the recovery of metal values (e.g., precious metals) entrained therein, which otherwise may have been lost.